The present invention provides systems and methods which can be employed to locate or detect presence of various materials, including nonferrous metals. These systems include new and useful sensors, circuits, systems and devices which power and/or interoperate with the sensors, and methods of making, operating and using such systems. Any or all of the systems, devices or processes can be combined with other systems, devices or processes disclosed.
Devices according to the present invention are capable of performing sophisticated target location, classification, and recognition of conductive or magnetic objects independently without the use of other devices or systems and can be deployed by themselves in an area survey. This is accomplished by the simultaneous and accurate phase and magnitude measurement of the response of a target object to time varying magnetic fields of several frequencies.
While several types of magnetic/electromagnetic methods have been employed by others, the ability to accurately measure phase and amplitude as a function of frequency has been lacking. This measurement is necessary for the classification of materials. Much of the most sophisticated magnetic target location research to date has been conducted by the United States Navy for the purpose of location and detection of mines and unexploded ordnance. This work has been carried out primarily by the Coastal Systems Station (CSS) of the Navy""s Naval Surface Warfare Center. They have developed some of the most sophisticated passive magnetic field sensing instruments. These include both total field and vector magnetometers as well as gradient magnetometers. However, their work does not include active time varying magnetic field generation.
Classical electromagnetic theory provides the underlying relationships for the description of operation of devices according to the present invention. An electrical current produces a magnetic field throughout space. The magnetic induction at any point in space, dB, due to a current flowing in an infinitely small (differential) length element is proportional to the magnitude of the current, I, times the vector cross product of the element length, dl, and the distance vector from the element to the point in space, x, divided by the cube of the magnitude of the distance vector:
dB=kcI(dlGx)/|x|3.xe2x80x83xe2x80x83(1)
where kC is the proportionality constant. In Gaussian units kC=1/c, where c is the speed of light. In MKSA units kC=1/(4xcfx80xcex50)1/2. Therefore, the total magnetic induction at any point in space can be calculated by integrating over the total current path for all currents of concern.
B=fkCI(dlGx)/|x|3.xe2x80x83xe2x80x83(2)
Furthermore, if the current is varying with time, then both I and dB become functions of time, I(t) and dB(t), and the relation becomes:
B(t)=fkCI(t)(dlGx)/|x|3.xe2x80x83xe2x80x83(3)
In free space the magnetic field, H, is proportional to the magnetic induction, B. The proportionality constant, xcexc0, is the permeability of free space and its value depends on the choice of the system of units. Thus:
H(t)=xcexc0B(t).xe2x80x83xe2x80x83(4)
The second relationship states that a time varying magnetic induction will produce an electric field, E, over any closed path as follows:
gE(t)xc2x7dl=xe2x88x92kE(d/dt)ffB(t)xc2x7nda.xe2x80x83xe2x80x83(5)
Equation 5 states that the line integral of E over the closed path whose elements are dl is proportional, kE, to the negative of the time derivative of the surface integral, over any continuous surface bounded by the closed path, of the vector dot product of the magnetic induction and the unit surface normal, n, to the surface enclosed by the path. da is a surface element of that surface.
Any material has an associated conductivity, "sgr"M, and magnetic permeability, xcexcM. Both the conductivity and permeability are material specific as signified by the subscript, M. These properties react with the local electric and magnetic fields, E and H. This interaction produces both currents and magnetic fields within the material. The magnetic induction in a material produced by the magnetic field is given by:
B(t)=xcexcMxcexc0H(t),xe2x80x83xe2x80x83(6)
where xcexc0 is the permeability of free space and xcexcM is the material""s relative permeability. There are three major classes of magnetic materials. They are differentiated by the size of xcexcM. The ferromagnetic materials have large, positive permeabilities. These materials include iron, nickel, cobalt and most of their alloys. The paramagnetic materials, which include most other metals, have permeabilities that are greater than one by only parts per million; their permeabilities are small when compared to those of ferromagnetic materials. The third class of magnetic materials is the diamagnetic materials. These materials have permeabilities that are slightly less than one by amounts equivalent in size to that of the paramagnetic materials. A few of the metals are diamagnetic. Ferromagnetic materials are referred to as ferrous materials and paramagnetic and diamagnetic materials are referred to as nonferrous materials.
The current by produced the imposed electric field, commonly called an eddy current, is proportional to the product of the impedance, ZM, which includes the conductivity and the shape functions, and the magnitude of the electric field, E; thus:
I(t)=k1E(t)/ZM,xe2x80x83xe2x80x83(7)
where k1 is the proportionality constant. The impedance is in general a complex number and is a function of the shape of the material along with its conductivity. As with magnetic materials there are three major classes of materials based on their conductivities. The conductors, which include metals, have relatively large conductivities. The insulators have conductivities that are a million to a trillion times smaller. In between lie the semiconductors. Materials can also be classified by their permitivities. In general the permitivities are relatively unimportant at the frequencies of interest for the conductors; therefore the effects due to the displacement currents are extremely small and can be neglected in the following arguments. Furthermore the displacement current will add a small xe2x88x92jZC term which will be overwhelmed by the much larger inductive term in the case of conductors. The aspect ratios of the targets are generally close to one. In this case for the materials of interest the impedance is strongly inductive and can be described by the relation:
ZM=ZR+jZL,xe2x80x83xe2x80x83(8)
where, ZR is the real impedance, which is a function of the conductivity and the shape, and ZL is the imaginary part of the impedance, which is a function of the shape.
The materials of interest have high permeabilities, high conductivities, or both. Equations 6 and 7 show that these materials will produce a secondary magnetic field either directly from the applied magnetic field due to xcexcM or indirectly through an induced current due to "sgr"M. It is these secondary magnetic fields that are sensed in all types of magnetic/electromagnetic sensing systems. Equation 5 shows that an electric field is only produced if the magnetic field is time varying; therefore, nonferrous materials cannot be detected by a constant magnetic field. The passive magnetic field sensing methods used by the Navy and all others for large area surveys are thus only capable of sensing the ferrous materials. In these methods the residual magnetic field from the earth, approximately 0.00005 T (Tesla) provides the local magnetic field at the target material. A ferrous target then creates a secondary field that can be sensed as a variation in the constant background field of the earth. This field is generally dipole in nature and falls off as the cube of the distance from the target. The field falls off very rapidly as the distance to the target increases. The field due to appropriate sized objects can be thousands or millions of times smaller than the background field of the earth. In addition the earth""s field varies with time. These variations although spatially uniform can be greater than the signal from the target material. In order to remove these variations, gradient techniques are used. The difference in the signal originating from two sensors physically separated will not contain any part of the uniform background, even if it is varying with time, if the sensors are perfectly matched. Only that portion of the signal originating from xe2x80x9clocalxe2x80x9d objects will be detected, due to their change as a function of position. The field from far away objects is much more uniform and will not be detected. In this manner the gradient technique is much more sensitive than the full field or vector magnetometers used in surveys.
There are drawbacks to using this gradient based system, however. The gradient decreases much more rapidly with distance than does the field. It decreases as the distance to the fourth power. The other concern is matching of the sensors. It is this matching that governs the degree to which the uniform (common mode) portion of the signal can be rejected. Systems can be built that are capable of achieving a factor of a million in common mode rejection. As good as these passive systems are they cannot detect nonferrous materials; however, in most situations, ferrous materials are associated with the nonferrous targets of interest and their detection can be used to map the debris pattern of an area.
In order to detect nonferrous materials, systems according to the present invention generate an alternating current in a transmitter coil. This current is of the form:
I(t)=Ixcfx89 sin(xcfx89t),xe2x80x83xe2x80x83(9)
where I( is the current magnitude and xcfx89 is the angular frequency xcfx89=2xcfx80f, where f is the frequency). Equations 3 and 4 thus state that there is a magnetic field at the target that is proportional to this current. Equation 6 shows that the secondary magnetic induction, Bf, produced by the target is proportional to this field; thus:
Bft)=kf sin(xcfx89t),xe2x80x83xe2x80x83(10)
where kf is the proportionality constant.
Equation 5 shows that there is an electric field at the target that is proportional to the negative of the time derivative of I(t). The time derivative of I(t) is given by:
dI(t)/dt=xcfx89Ixcfx89 cos(xcfx89t);xe2x80x83xe2x80x83(11)
thus the electric field impressed onto the target, ET, is:
ET(t)=xe2x88x92kExcfx89Ixcfx89 cos xcfx89t),xe2x80x83xe2x80x83(12)
where kE is the proportionality constant.
If the material is highly conductive as are most materials of interest then the character of its impedance is mostly imaginary and appears as an inductance of the form:
ZL≈jxcfx89L,xe2x80x83xe2x80x83(13)
where L is the effective inductance of the sample. This imaginary impedance adds an effective time integration to the electric field to produce the induced current. Thus the induced current, IT, at the target is:
IT(t)=xe2x88x92k1 sin(xcfx89t),xe2x80x83xe2x80x83(14)
where k1 is a constant of proportionality. Equation 3 gives the secondary magnetic induction as proportional to the current. Thus the secondary induction due to the conductivity of the target, Bn, is:
Bn(t)=xe2x88x92kn sin(xcfx89t),xe2x80x83xe2x80x83(15)
where kn is the proportionality constant. Equations 10 and 15 show that the contributions to the secondary fields at the sensors due to the magnetic and electrical properties of the materials would be opposite in sign. It is therefore possible for an active AC system of this type to differentiate between ferrous materials which are dominated by the Bf term and the nonferrous materials which are dominated by the Bn term. The total field at the sensors is:
Btot(t)=Bf(t)+Bn(t).xe2x80x83xe2x80x83(16)
In the simplified case described above the total field is given by:
Btot(t)=(kfxe2x88x92kn)sin(xcfx89t).xe2x80x83xe2x80x83(17)
Thus the signal varies from positive to negative depending on the relative strengths of the magnetic and electrical properties of the materials.
The active DC magnetic field sensing methods create a time varying magnetic field by moving a magnet across the field to be surveyed. This creates a time varying magnetic field at a given any given point in the field. In this manner the active DC magnetic field sensing technique can be used to differentiate between ferrous and nonferrous materials.
The limitation of the moving magnet type of system is that if there is a mixture of ferrous and non ferrous material, the one with the stronger secondary field will dominate. Only that material will be detected, but at a weaker signal strength than if it were present alone. It would be possible to mask the nonferrous signal with ferrous material. The active system overcomes this limitation. A more detailed analysis of the material impedance shows a complex character that can be used to better differentiate between materials. Equation 8 shows the material impedance as composed of a real part, ZR, and an imaginary part, ZL. The above approximation assumed that because the conductivity of the material was large that the real part of the impedance was near zero and could be neglected. This is true only to a first order approximation. In a real material the real part of the impedance is a factor. Not only does the material exhibit a real impedance but its magnitude is dependent on the permeability, conductivity and frequency. The real part of the impedance is proportional to the conductivity. In addition for an AC magnetic field the penetration into an object is limited by an attenuation factor. To first order approximation there is a surface layer in which all conduction can be considered to take place. This layer restricts the current flow. Its thickness is referred to as the skin depth, xcex4, which has the form:
xcex4=[2/xcfx89xcexc0xcexcM"sgr")]1/2.xe2x80x83xe2x80x83(18)
The real part of the impedance can be expressed as:
ZR=kR/{"sgr"[2/xcfx89xcexc0xcexcM"sgr")]1/2},xe2x80x83xe2x80x83(19)
which reduces to:
ZR=kR(xcfx89xcexc0xcexcM)1/2/(2"sgr")1/2.xe2x80x83xe2x80x83(20)
The impedance has p phase, xcfx86Z, that is given by:
xcfx86Z=tanxe2x88x921(ZL/ZR).xe2x80x83xe2x80x83(21)
Thus Equation 14 becomes:
IT(t)=xe2x88x92k1 sin xcfx89(t+xcfx86Z),xe2x80x83xe2x80x83(22)
and by substitution into Equation 16, the magnetic field, due to the material, as seen at the sensor is given by:
Btot(t)=kf sin(xcfx89t)xe2x88x92kn sin(xcfx89t+xcfx86Z).xe2x80x83xe2x80x83(23)
The phase of the sensor signal at the target due to the secondary field generated by the target material has a phase which is no longer merely positive or negative. A measure of this phase is material specific. This phase is a function of the material conductivity and permeability and of the applied magnetic field frequency. A measure of the relative phase between the applied magnetic field and the secondary field generated by the target as a function of frequency gives a material specific signature which can be used to differentiate one material from another. In order for a system to make use of this feature it must be capable of accurately measuring the relative phases. The active DC magnetic field techniques, as practiced by others, of passing a magnetic field over a target, cannot accomplish this since there is no phase reference for the applied field.
The active AC magnetic field generated by this system is capable of precise phase measurement. Since the active system generates a precise AC magnetic field and has the waveform available for comparison, it can use synchronous detection to precisely determine the phase and amplitude at any frequency and to reject noise both out of band and in band uncorrelated noise. The system in fact can generate multiple frequencies simultaneously and can synchronously detect the secondary fields generated by the target materials simultaneously at each frequency. The use of multiple frequencies all of which are exactly correlated achieves an extremely low dynamic noise level. These multiple frequencies are also preferably digitally time encoded to further eliminate background noise. This time encoding involves periodically in time changing the phase of the transmitted frequencies. The encoding can be designed so that the temporal correlation of a non correlated signal will be very close to zero; whereas the correlation of the transmitted signal with the target signal will remain unchanged and large.
This system can suffer background and noise limitations similar to those of the passive magnetic field techniques. For that reason the preferred active system employs multiple matched sensor sets, positioned symmetrically to the transmitter field, to simultaneously measure in multiple directions both the magnetic field components and its gradients. The use of gradients greatly increases the system""s immunity to background noise from distant objects and from slowly varying backgrounds. In this way the far field electromagnetic noise from the earth is eliminated along with the broad conductivities of mineralization. In addition the relative structure of the field gradients can be used to determine distance and direction of the target from the sensor system, not only providing desired target information but also providing a means of eliminating any signals from the tow vehicle.
The precision of the background field elimination in the active system is different from that of the passive systems. In the passive systems the background field of the earth must be eliminated from the sensor signals. In the active system the field due to the transmitter coil must be eliminated from the sensor signals as well. While the earth""s background signal is approximately 0.00005 T the field due to the source can be as high as a few Tesla or ten thousand times greater than the earth""s field. Because the output of the transmitter is known, synchronous system techniques can be used along with the shape of the magnetic field transmitter to eliminate this field from the sensors.